Potassium flux in smooth muscle of frog stomach

In frog stomach muscle fibers, normal steady-state K flux, estimated directly from 42K uptake, was 0.17 pmol/cm2 per s at 5 degrees C and 0.63 pmol/cm2 per s at 15 degrees C. Influx characteristics were studied at 5 degrees C, where backflux and diffusional delay effects are minimized. Steady-state K influx was a saturating function of external [K] over the range 0.25-11 mM [K]o; influx at normal and higher [K]o did not differ significantly. Na loading (in K-free or low K solution) strongly stimulated influx, which showed altered saturation kinetics; maximal K influx was a quasilinear function of internal [Na]. Ouabain (10(-4) M) reduced normal and stimulated K influx markedly. Ethacrynic acid (10(-3) M) caused net K loss and Na gain, but increased K influx fourfold; ouabain inhibited the stimulated influx by 50%. These results indicate that K influx depends mainly on cycling of the Na-K pump and is normally limited by Na efflux. Ethacrynic acid may stimulate another mode of pump operation, K-K exchange, and uncouple the normal operation.